In a corrosive environment containing carbon oxide and a very small amount of hydrogen sulfide, a 13% Cr martensitic stainless steel is normally used, because such an environment requires excellent properties regarding the corrosion resistance, the stress corrosion cracking resistance, the weldability, the toughness and the mechanical strength as for a steel material. Specifically, API-13% Cr steel (13% Cr-0.2% C), which is specified according to the standard of the API (American Petroleum Institute), is widely used in such an environment, because it has an excellent corrosion resistance to carbon dioxide. The API-13% Cr steel can be used as a material for a conventional oil country tubular goods which require a mechanical strength of order of yield stress 552-655 MPa (80-95 ksi). However, API-13% Cr steel has a relatively small toughness and therefore cannot be used as a material for a deep oil well steel pipe which requires a much greater mechanical strength of order of yield stress more than 759 MPa (110 ksi).
In recent years, improved type 13% Cr steel, which includes carbon in an extremely reduced amount and which includes Ni instead of carbon, has been developed to improve the corrosion resistance. Since the improved type 13% Cr steel provides an excellent toughness even in an increased mechanical strength and therefore can be used in a much severer corrosive environment, it is increasingly used in an environment requiring a high mechanical strength. However, a decrease in the C content tends to provide the precipitation of δ ferrite, which are harmful for the hot workability, the corrosion resistance, the toughness and the like as for steel. As a result, an appropriate amount of Ni, which is considerably expensive, has to be included in the steel in accordance with the amounts of both Cr and Mo added, thereby causing its price to be considerably increased.
In order to overcome such a problem, several attempts have been made to improve the toughness in the 13% Cr steel having a high mechanical strength. For instance, in Japanese Patent Application Laid-open No. 8-120415, an attempt has been made to improve the mechanical strength and the toughness on the basis of API-13% Cr steel, using active N which cannot be immobilized by Al. However, the 13% Cr steel in the prior art has an yield stress of 552-655 MPa (80-95 ksi) and a fracture appearance transition temperature of −20 to −35° C. in the Charpy impact test, as described in the examples of the embodiments, so that the toughness cannot be obtained even in a high mechanical strength of more than 759 MPa (110 ksi).
On the other hand, a number of technologies have been disclosed to use the retained austenite in order to improve the property of 13% Cr steel. In Japanese Patent Application Laid-open No. 5-112818, a technology is disclosed for thermally refining 13% Cr steel to provide a low mechanical strength and a high toughness through the precipitation of coarse carbide particles in a martensite structure having a high carbon content, wherein the heating in a dual phase region is carried out prior to the annealing to segregate carbon in an austenite phase newly generated in prior austenite grains and then the annealing treatment is carried out.
In Japanese Patent Application Laid-open No. 8-260038, a technology is disclosed for thermally refining a 13% Cr steel to provide a low mechanical strength and a high toughness by weakening the solution strengthening effect, wherein C and Ni in the austenite are enriched by heating in a dual phase region and thereby reduces the C and Ni contents in the martensite as a parent phase.
However, these technologies are used only to thermally refine the 13% Cr steel so as to securely provide a low mechanical strength and a high toughness, but provide no means for increasing the mechanical strength and the toughness by improving the property of the 13% Cr steel.
Moreover, a technology has been disclosed to obtain a steel having a high mechanical strength and a high toughness by utilizing the retained austenite in the steel. In Japanese Patent Application Laid-open No. 11-310823, a technology for obtaining a high mechanical strength and a high toughness wherein a 13% Cr steel containing carbon is heated in a dual phase region at Ac1-Ac3 to form reverse transformed austenite in the parent phase of martensite, and a tempering treatment is then performed at a temperature of lower than Ac1. In the specification, however, no reference is made for the technology providing a steel material having such a high mechanical strength as yield stress of greater than 759 MPa (110 ksi), which is required for developing deep oil wells.
In Japanese Patent Application Laid-open No. 2000-226614, furthermore, a technology for providing a high mechanical strength and a high toughness has been disclosed, wherein the heating in a dual phase region is carried out at Ac1-Ac3 in an improved type 13% Cr steel having a low carbon content so as to form austenite in the parent phase of martensite. However, although it is sure that the steel disclosed therein provides a high toughness, a greater content of expensive nickel is used and also the thermal treatment is carried out in a restricted control range in order to precipitate the retained austenite. Accordingly, there exists a problem that the price of the steel is greatly increased, compared with the API-13% Cr steel.
As described in the above-mentioned Japanese Patent Application Laid-opens No. 5-112818 and No. 2000-226614 respectively, it is known that the existence of retained austenite in the steel provides an improvement of the toughness in the 13% Cr steel. On the other hand, it is also known that the existence of retained austenite in the steel reduces the mechanical strength (for instance, Japanese Patent Application Laid-open No. 8-260038). Consequently, it can be assumed that the existence of retained austenite in the steel improves the toughness of the steel, but at the same time reduces the mechanical strength.
Moreover, as described in the above-mentioned Japanese Patent Application Laid-opens No. 11-310823 and No. 2000-226614, the method for producing the steel having a high mechanical strength and a high toughness by utilizing the retained austenite is demonstrated. Nevertheless, the method has not yet disclosed capable of obtaining the steel material, which has such a high toughness and provides such a reduced cost as applicable to the development of oil wells requiring an yield stress of greater than 759 MPa (110 ksi).